1. Field of the Invention
The present invention relates generally to determining a signal detection threshold for optical signals in an optical network. More particularly, the present invention is directed towards adjusting an optical signal detection threshold appropriately for different network conditions.
2. Description of Background Art
Optical data networks typically include two or more optical nodes linked by optical fibers into a network. Each node may include one or more optical transmitters, optical receivers, optical amplifiers, and optical multiplexors along with one or more node controllers. The network may be one of several common topologies, such as a linear chain network, an optical mesh network, or an optical ring network.
Typically, the optical network is designed using a power budget analysis of the optical path. In a power budget analysis, the optical power at the receiver and its signal-to-noise ratio must be sufficiently large to achieve a desired bit error rate (BER). This can be expressed mathematically as: lm>Pt(dBm)−Plosses (dBm), where lm is a margin required to achieve a BER given the electrical and optical noise at the receiver, Pt is a transmitted power, and Plosses are the net optical losses along the portion of the optical path from the optical transmitter to the receiver, which includes the optical attenuation of passive optical fibers and components offset by the amplification of any optical amplifiers along the optical path. The use of optical amplifiers along the optical path (including pre-amplifiers and post-amplifiers) reduces the effective optical losses but also injects optical noise.
Each receiver within an optical node typically includes at least one optical detector (also commonly known as a “photodetector”) for recovering the data stream of a data channel. Additional photodetectors may be included in an optical node to perform a monitoring function of one or more data channels. For example, dense wavelength division multiplexing (DWDM) nodes may include an optical receiver for each wavelength channel coupled to a tributary network along with additional optical detectors for monitoring the behavior of two or more wavelength channels communicated with a neighboring node.
One way that an optical detector is used to monitor an optical input is known as a “signal detect” function. FIG. 1 is a block diagram of a conventional optical receiver 100 having a photodetector 120, front-end electrical amplifier 130, and decision circuit 140. An optical pre-amplifier 110 may be coupled to optical receiver 100. Decision circuit 140 is used to recover an optical data stream. Additionally, front-end amplifier 130 may include a peak-detector 135. The output of peak-detector 135 is used to provide a signal detect (SD) output. The SD output is typically a logical “1” if the measured peak optical power level exceeds a pre-selected signal detect threshold power level and a logical “0” otherwise.
The SD output is often used as part of a fault detection and restoration system that monitors potential faults in the optical network and initiates restoration events in the event of a fault, such as switching to redundant electrical components or performing a line switch to a protect fiber. If the SD output is used for fault detection and restoration, it is important to avoid network conditions that could result in an erroneous SD output.
A conventional front-end amplifier 130 with a peak-detect circuit 135 provides a comparatively inexpensive circuit to provide a SD output. However, a drawback of receiver 100 is that the SD output will only be reliable if the optical signal and optical noise levels remain within specific ranges with respect to the threshold peak power level. In particular, the operation of the network is constrained to a set of conditions corresponding to: 1) preventing the received peak optical power level of data signals from dropping below the signal detect threshold power level for all possible optical attenuation levels; and 2) preventing the received peak optical power level of noise level for a loss-of-signal condition from rising above the signal detect threshold power level for all possible optical attenuation levels. These conditions, in turn, may require costly field calibrations of the signal detect threshold power level or impose undesirable limitations on network design and operation.
Therefore, there is a need for a new system and method of signal detection in an optical network.